Daikin 8A Fault Code (8A-46): Freeze Protection Activated
The Daikin 8A fault code — displayed in full as 8A-46 in Daikin's documentation — indicates that the boiler's freeze protection mode has been triggered. This happens when the flow temperature sensor detects a reading below 1°C, causing the boiler to suspend normal operation as a precautionary safety measure. In genuinely cold conditions this is the system working as designed; in milder weather it may point to a faulty sensor giving an incorrect reading.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Daikin 8A fault code mean?
The Daikin 8A fault code — displayed in full as 8A-46 in Daikin's documentation — indicates that the boiler's freeze protection mode has been triggered. This happens when the flow temperature sensor detects a reading below 1°C, causing the boiler to suspend normal operation as a precautionary safety measure. In genuinely cold conditions this is the system working as designed; in milder weather it may point to a faulty sensor giving an incorrect reading.
Common causes
- Genuine freezing conditions with frost protection disabled Common
The most common trigger. If the boiler has been switched off or left running without frost protection enabled during a cold snap, the flow temperature can drop below 1°C and activate the 8A-46 code. This is the boiler behaving exactly as intended, not a component failure.
- Faulty flow temperature sensor Sometimes
If outdoor temperatures are above freezing when the fault appears, the flow temperature sensor is likely sending an incorrect reading to the boiler's control board. A failed or drifting sensor can falsely report sub-1°C conditions and trigger freeze protection mode unnecessarily.
- Faulty or miscalibrated thermostat Rare
In some cases the room thermostat or system controller sends incorrect temperature data that, combined with other conditions, contributes to a spurious 8A-46 activation. Less common than a direct sensor failure but worth noting if the sensor tests healthy.
How to fix it
- Check the outdoor temperature and recent weather conditions DIY safe
Before doing anything else, consider whether overnight or daytime temperatures have been at or below freezing. If they have, the 8A-46 code is most likely the boiler's protection system doing its job correctly, and no repair is needed — just work through the following steps.
- Reset the boiler using the controls on the unit DIY safe
Perform a standard reset as described in your boiler's user manual — typically a button hold or menu selection. A single reset attempt can clear the fault if the flow temperature has now risen above the 1°C threshold. Avoid resetting more than two or three times; repeated resets without a successful restart suggest a deeper issue.
- Enable the frost protection setting on your boiler or controller DIY safe
Navigate to your boiler's frost protection or anti-freeze mode within the user settings menu and switch it on. This keeps the system circulating water at a minimum safe temperature during cold periods, preventing the flow temp from dropping to the trigger point again. Consult your user manual for the exact menu path — it varies by controller model.
- Keep the heating on a low background setting during cold spells DIY safe
Rather than turning the boiler completely off during very cold weather, set it to a low background temperature (around 15–16°C) when the property is unoccupied. This is the most practical way to prevent the 8A-46 code from recurring each winter.
- Call a Gas Safe registered engineer if the fault returns in mild weather Gas Safe engineer
If the 8A-46 code appears when outdoor temperatures are clearly above freezing, and resetting the boiler does not clear it, the flow temperature sensor is the most likely culprit. A Gas Safe registered engineer should inspect the sensor, test its resistance readings, and replace it if it is producing inaccurate data. Do not attempt to access or replace internal sensors yourself.
Parts you may need
- Flow temperature sensor (NTC thermistor) · from £25
- System controller / thermostat unit · from £85
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£320, depending on the underlying cause.
Frequently asked questions
Is the Daikin 8A fault code dangerous?
No, in most cases it is not dangerous. The 8A-46 code is a protective safety feature that stops the boiler operating when the flow temperature drops to near freezing, preventing potential freeze damage to pipework and components. It is more of a warning than a critical fault. That said, if the code appears repeatedly without a cold-weather explanation, you should have a Gas Safe engineer investigate a possible sensor fault rather than continuing to reset the boiler.
Can I fix the Daikin 8A fault code myself?
Partially. If cold weather is the cause, you can clear the fault yourself by resetting the boiler and enabling frost protection mode in the user settings — no engineer needed. However, if the fault is caused by a faulty flow temperature sensor, that component sits inside the boiler and must be inspected and replaced by a Gas Safe registered engineer. Attempting to work on internal boiler parts yourself is unsafe and could invalidate your warranty.
How much does it cost to fix a Daikin 8A fault caused by a bad sensor?
For most households, the total bill for a flow temperature sensor replacement sits between £120 and £320, covering the engineer's call-out, an hour or so of labour, and the sensor itself (which typically costs around £15–£30 for the part alone). Weekend or emergency call-outs will push the cost toward the higher end. In the rare situation where the fault has damaged the PCB or another major component, costs can be significantly higher — worth discussing with your engineer before authorising work.
How do I stop the Daikin 8A code coming back every winter?
The most effective prevention is to switch on the built-in frost protection mode via your boiler or system controller settings — this keeps water circulating at a minimum safe temperature whenever the system detects very cold conditions. As a backup, avoid turning the boiler completely off during periods of freezing weather; a low background temperature setting of around 15°C is enough to keep the flow temperature safely above the 1°C trigger point.